In this major work, originally published in 1974 and reporting on over ten years' research of the Geneva School, the authors carried the pioneering investigations of Jean Piaget to a new and remarkable level. As Piaget said in his foreword to the book: 'The novelty of the findings, the clarity of the theoretical interpretation, and the sometimes even excessive caution of the conclusions enable the reader to separate clearly the experimental results from the authors' theoretical tenets.'